Wind turbine blades and many other large composite structures such as composite aircraft wings, fuselages and boat hulls, are typically formed in large female moulds using composite fabrication techniques such as vacuum assisted resin transfer moulding (VARTM). The process involves arranging one or more layers of dry glass-fibre fabric in the mould together with other structural components to form a ‘lay-up’.
The lay-up is then covered by vacuum film and sealed against the mould to create a sealed region enclosing the lay-up. Air is removed from the sealed region to create an effective vacuum and resin is admitted into the sealed region. The resin infuses through and between the glass-fibre layers and other structural components in the sealed region. Heat is then applied to the resin-infused lay-up to cure, i.e. harden, the resin. The cured resin serves to integrate the various layers together in the composite structure.
The moulds for modern wind turbine blades are very large, in some cases in excess of 80 metres in length, and several metres in width. Such large moulds are naturally very expensive and occupy a significant amount of space within a blade manufacturing facility. Accordingly, a typical blade manufacturing facility may only have a single mould, or just a few moulds. In order to optimise the efficiency of the manufacturing process and increase the rate of blade production, it is desirable to minimise the ‘in-mould time’ associated with the process, i.e. the amount of time that a mould is monopolised in order to make a blade.
In order to minimise the in-mould time of blade production, some operations are performed offline, in advance of the mould being used. For example, multiple glass-fibre layers or ‘plies’ may be stacked together on a flat surface such as a table or the factory floor to form a stack (also referred to as a ‘kit’ or ‘pack’). The stacked layers are held together by stitching or other suitable means such as adhesive tape. The pre-assembled stack is then transferred to the mould and arranged in the mould at the appropriate time in the manufacturing process. By forming the stack offline, multiple plies can be arranged in the mould in a single operation, whereas previously each ply would have needed to be arranged in the mould individually. The use of such pre-assembled stacks can therefore significantly reduce the in-mould time required to make a blade.
In a typical wind turbine blade, there are different numbers of stacked glass-fibre layers in different regions of the blade, according to the structural requirements of those regions. For example, the root of the blade is required to be very thick and hence ten or more glass-fibre layers may be stacked in the root region of the mould. In contrast, the tip of the blade is very thin and may only require a single glass-fibre layer. Other discrete regions of the blade may require extra layers of glass-fibre fabric where local reinforcement is required. These factors are all taken into consideration when preparing the stacks, so a stack may vary in thickness across its width and along its length in some cases.
Presently, the stacks are stored in a generally flat state, and lifted and placed in the blade mould when required. However, the flat stacks are difficult to store and take up significant space. They are also difficult to handle and several operators may be required to support the periphery of the stack as it is lifted and placed in the mould. As it is desirable to produce wind turbine blades of increasing size (in order to capture increasing energy from the wind), it is important that manufacturing processes can be scaled up. The present method of storing and placing stacks in the mould is not easily scalable as handling and storage problems are exacerbated with increasing stack sizes. Therefore, the present method is only suitable for relatively small stacks, and hence very many stacks must be assembled and arranged individually in the mould to make a large wind turbine blade.
Referring to FIGS. 1a-1c, in seeking to develop a scalable solution that addresses the above problems, and which further reduces the in-mould time of blade production, the inventors of the present invention conducted non-public trials in which a plurality of glass-fibre strips 12a, 12b were stacked one on top of another to form a stack 10. The stacked strips were stitched together along stitch lines 13 at regular intervals along their lengths (FIG. 1a). The stack 10 was then rolled around a cylindrical drum 14 of circular cross section to form a roll 16 (FIG. 1b). For ease of illustration, the stack 10 shown in FIGS. 1a-1c consists of two glass-fibre layers 12a and 12b, which are stitched together, but in reality the stack may comprise several layers stitched together. Also, whilst the roll 16 shown in FIG. 1b comprises only a single turn of the stack 10 around the drum 14, in practice the stack 10 is turned several times around the drum 14.
The roll 16 addressed the storage problems associated with large flat stacks as the roll 16 takes up less space in the factory than flat (i.e. non-rolled) stacks. Handling and placement is also facilitated in comparison to the prior art method as the roll 16 is simply unrolled in the mould when required, which can be performed by relatively few operators.
However, it was noted that the stacked layers 10 had a tendency to develop wrinkles when formed into a roll. Referring to FIG. 1c, which is a schematic drawing showing the stacked strips 12a, 12b being rolled around part of the drum 14, the cause of the wrinkles 15 was found to be attributable to the outer layers 12b of the stack 10 having a larger radius of curvature, and hence a longer circumferential length, than the inner layers 12a of the stack 10 when rolled around the circular drum 14. This causes different levels of tension in the various layers 12a, 12b of the stack 10 and causes some of the layers to move or stretch more than other layers. As relative movement between the layers 12a, 12b is constrained by the stitches at the stitch lines 13, one or more of the layers 12a, 12b inevitably develops wrinkles 15 between the stitch lines 13 due to the circumferential path difference between the layers 12a, 12b. In this example, the outer layer 12b of the stack 10 has a longer circumferential path around the drum 14 than the inner layer 12a and hence the inner layer 12a tends to develop wrinkles 15 between the stitch lines 13.
The problem of wrinkles developing was found to be particularly acute for stacks consisting of several layers, such as the stacks required for relatively thick parts of the wind turbine blades, where eight or more glass-fibre layers are stitched together to form the stack. It is desirable to avoid wrinkles in the fibre layers as wrinkles tend to compromise the structural integrity of the finished component.
Accordingly, it is an object of the present invention to provide a scalable solution that avoids or at least minimises the formation of wrinkles in the fibrous layers.